Writing information to a magnetic storage medium includes generating a magnetic field in close proximity to the storage medium to be written. In conventional storage devices using a magnetic medium, the magnetic field is generated in close proximity to the magnetic storage medium using a conventional read/write head assembly. The read/write head assembly can include inductive write and magneto-resistive (MR) read elements. Information to be stored is sent to writing/encoding circuits. The writing/encoding circuits encode the information to maximize storage efficiency. The writing/encoding circuits then modulate a current in the write head to produce a magnetic field of alternating polarity that magnetizes the storage medium. The quality of the written information is highly dependent on a proper spacing (i.e., fly-height) between the write head and the medium.
Referring to FIG. 1, a diagram is shown illustrating a read/write head assembly 10 disposed in relation to a storage medium 12 as a way to depict a magnetic fly-height (or spacing) 14. The distance between the read/write head assembly 10 and the storage medium 12 is commonly referred to as the fly-height. Proper control of the fly-height during reading is required to assure that the read back signal exhibits the best possible signal-to-noise ratio, and thereby improves performance and prevents injurious head-disc contact. In general, the term fly-height is used to refer to the magnetic fly-height 14. The magnetic fly-height 14 generally corresponds to a distance between a magnetic film on the storage medium 12 and transducer pole-tips of the read/write assembly 10. However, because the head surfaces of the read/write assembly 10 and the storage medium 12 are protectively overcoated and lubricated (i.e., coating layers 16 and 18, respectively) to eliminate corrosion and mitigate damage from momentary head-disc contact, a physical fly-height (or spacing) 20 is less than the magnetic fly-height 14 by the aggregate thickness of the coatings 16 and 18.
In a conventional disk file, fly-height is determined by measuring amplitudes of two or more harmonics of the readback signal. The conventional approach uses a vacant or dedicated area on the magnetic storage medium containing a periodic pattern from which the harmonics can be measured. While the conventional approach provides a reasonably static estimate of fly-height, the conventional approach does not provide an indication of any changes in fly-height occurring during standard operational periods. As such, the conventional approach does not provide an ability to adjust for changes occurring during the operation of the magnetic storage medium. To mitigate partially the inability of conventional fly-height measurement schemes to control fly-height during the course of a long write or read data transfer, servo information interleaved with the data can be used as a source of read back harmonics.
Conventional harmonic-amplitude-sensing based fly-height measurement methods rely on precise knowledge of the gain of the readback signal path comprising Preamplifier and Recording Channel analog circuits and the interconnecting transmission lines. Other methods of fly-height measurement, for example those based on channel bit density (CBD) estimation and on amplitude of the overall readback signal, can exhibit sensitivity to gain variation at specific frequencies and hence can benefit from an ability to stabilize gains at those frequencies.
Fly-height measurement accuracy is limited by unavoidable drifts in amplitude response of the preamplifier reader, and in the read channel analog portions. Even in a state-of-the-art device, accuracy of fly-height measurement below ˜4 nm spacing is poor. A further tolerance-related issue arises in Bit Patterned Media (BPM) Recording in which precise alignment of written transitions to predeposited lands on the storage medium is sought. In this mode of recording, compensation of delay changes in the read and write paths encompassing the preamplifier and record channel is crucial. One method of compensating delay time variation uses periodic iterative write/read operations in a predefined area to determine a write phase yielding maximum playback amplitude. However, such a method degrades average file transfer rates.
A method and/or apparatus for measuring and/or compensating variation in preamplifier frequency response in order to maintain constant relative gains over device lifetime, at specific frequencies, would be desirable. It would also be desirable in BPM recording systems to provide a method and/or apparatus for measuring aggregate delay of preamplifier and channel write and read paths, to allow compensation of variation in the delay.